An entirely new reason to dislike MIM

[Candiru]

Prior to last night, I had a live and let live attitude toward parts made with metal injection molding: As long as they worked well, I would just consider them another kind of metal.

I ended up buying a no-name thumb safety, as it was just about the only way to find a safety that wasn't long and wide enough to land an F-18 (that must be what makes them tactical). The safety had some really sharp edges that ended up raising a blister on my thumb, so I chamfered the edge; did a pretty good job, too, by my standards. This left some exposed metal, so last night I broke out the cold blue to touch it up.

But it won't take bluing! I sprayed the thing with enough carb cleaner to degrease a tank and cleaned with denatured alcohol, but no amount of cold blue would make that shiny metal dark. I thought I was going insane until it occurred to me that MIM might be to blame. It looks like the previous finish was some sort of spray-on deal, so there goeth I. I'm thinking some Rustoleum might do the trick--it won't take blue, but I'm sure it'd be happy to rust up.

Just thought I'd share this story in the hopes it might prompt a grin or two.

[TonyW]

Just a guess, but maybe that part wasn't steel MIM? Non-ferrous metals can be used in MIM.

[gbw]

Or black coated stainless?

[Ten_Ring]

I ended up buying a no-name thumb safety

That is most likely your answer. There is no telling what it is made of. I have seen aluminum, stainless, carbon steel and some othe odd one.
What is the weight like? is it heavy for the size...light. Will a magnet stick to it? Even low grade stainless is magnetici.e. 304,309. If it's light it may be aluminum or a type of pot-metal.

[500 Magnum Nut]

MIM can be any type of metal, stainless, alumium etc.
What makes the blue work is the carbon in the steel. Your part is lacking carbon...

[Lubaloy]

Aluminum cannot be MIM'ed.
300 series stainless steel is not 'low grade' nor is it magnetic.
Most Kimber small parts are MIM'ed stainless steel, coated black.
:)

[Ten_Ring]

Not to start any type of argument, but I work with 304, 309, 316 every day and have been for the past 17 years. Metallurgy was a requirement for this job. 304 & 309 are magnetic they have a higher corbon content than 316. To prove my point....stainless will rust, why..... the carbon.

[Involute]

Jumping into the un-started argument...

Austenitic stainless (300 series) is generally non-magenetic, but can become magnetic through cold working processes.

Martinsitic stainless (400 series) is magenetic and generally can rust.

Chrome content is what protects stainless from rusting, the iron is what does the rusting (oxidizing), or bluing as the case may be, carbon mostly affects the hardenability and has little effect on the oxidation process.

[Ten_Ring]

Involute,
Very well put. When I say magnetic, I do not mean to the point of a "strong" pull, rather just enough to feel the pull with a good magnet.

[500 Magnum Nut]

Here is an explanation by Herb Belin of Smith and Wesson on MIM parts. I first saw this posted by Victor Louis (S&W FORUM), and saved it.


By popular request, here's the post from Mr. Herb Belin of S&W -----------------------
"I have read with much interest the many comments in this forum pertaining to MIM, MIM Parts and the use of same in a S&W product. So far I have come away with several impressions and they are "people in general don't like/trust MIM parts" and "no one has said why" I will take a stab at this issue and see where it goes.

As background to our decision to use MIM in some areas of our Mfg Process we took a long hard look at our "Life Time Service
Policy". It was clear to us that any change in any of our products such as the use of MIM components had to show equivalent or better performance and durability to those components that were being replaced or the "Lifetime Service" would haunt us forever. The second consideration was to determine if the change was too radical a departure from S&W mainstream design.

For the performance and durability issues we decided that if MIM could be used for the fabrication of revolver hammers and triggers successfully this would truly be an "Acid Test". There is nothing more important to a revolvers feel than the all-important Single Action Sear that is established between the hammer and the trigger. Mechanically few places in a revolver work harder than at the point where the hammer and trigger bear against each other. If these surfaces wear or loose there "edge" the "feel" is lost. Initial testing was on these two critical parts. Over time we arrived at a point where our best shooters could not tell the difference between a revolver with the old style hammer and trigger and the new MIM components. Special attention was given to their endurance when used in our very light Magnum J frames such as the early prototype 340 & 360 Sc's. None of our revolvers work their components harder than these small magnum revolvers. Throughout this testing MIM held strong and finally we determined that this change judged on the basis of durability and feel was a good one.

The second area of concern to S&W was our customer’s reaction to this departure from the traditional. Many heated, intense discussions resulted but in the end the decision was made to move ahead with MIM.
The issue of cost was only one of the considerations in making this decision. Equally as important was the issue of part-to-part uniformity and the result of this of course is Revolver-to-Revolver consistency. We found that revolvers that used MIM hammers and triggers required almost no Fitter intervention in those areas during final assembly and final inspection and Trigger Pull Monitor rejection rates dropped markedly on finished guns. From an internal process point of view it appeared a "Winner".

Lets shift gears for a moment and talk about the MIM process. It is unclear to me as to the reason for many of the negative feelings on the forum concerning MIM. Typically when people complain and aren't specific in the reason why, the problem is often created by a departure from the "Traditional". Perhaps that is indeed what is bothering some people when they view MIM.

The term MIM stands for Metal Injection Molding. It holds some similarities to Plastic Injection Molding and many differences as well. To start we would take a finally divided metal powder. This could be stainless or carbon steel. Today even Titanium is being used in some MIM fabrications. We would mix the metal powder and a thermoplastic binder (generally a Wax) forming slurry of sorts when heated and inject this mix into a precision mold and finally form what is known as a “Green Part". This part is roughly 30% larger than the finished part it will become at the end of the process. Interestingly enough the Green Part at this stage can be snapped in two with simple finger pressure. The Green Parts are then placed in a Sintering furnace filled with dry Hydrogen gas and the temperature is brought almost to the melting point of the metal being used. Over time the "Wax" in the Green Part is evaporated, the metal fuses and the part shrinks 30% to it's final correct dimensions. At this stage of the process the MIM part has developed 98 to 99%of the density of the older wrought materials and a metallurgy that is almost identical. Dimensionally it is finished and no machining is required. However the job is not yet done and the MIM parts are brought to our Heat Treat facility for hardening and in the case of Hammers and Triggers, Case Hardening. Depending on the particular metal alloy that was used at the start of the process we apply a heat treat process that is the same as would be used if the material were the older wrought style. Final hardness, Case thickness and core hardness are for the most part identical to parts manufactured the older way.

Lets look for a moment at how we achieve dimensional precision when comparing these 2 processes. The old parts were each machined from either bar stock or a forging. Each cut and every resulting dimension was subject to machine variations, Cutter wear, operator variations etc. If every operation was done exactly right each and every time and the cutter didn't let you down you would have produced a good part but sometimes this didn’t happen resulting in a rejected gun and rework or in the worst case an unhappy customer. With MIM parts you must still machine to very high tolerances and your cutters have to be perfect and your machinist has to be highly qualified but all of this only has to come together one time. That time is when the injection mold is made. Typically a mold for this process costs S&W between 30,000 and 50,000 dollars. Once it is perfect every part it makes mirrors this perfection and you have in my view a wonderful manufacturing process.

Hopefully this description will help us all better understand the MIM process.
Please forgive the spelling errors and misplaced punctuation. I have no spell checker on this and the phone continues to ring!

Have a Great Weekend,
Herb

Additional Point.
Currently S&W is paying about $1.20/Lb for stainless steel bar stock. Raw MIM stainless steel inject able material costs $10.00/Lb."

[groundpounder]

Great post Magnum!

[Candiru]

Well, the thumb safety is magnetic, so it must be stainless steel with paint on it. Well, two can play at this game: I swung by the hardware store and picked up a can of Rustoleum's barbecue paint, which will hopefully approximate the texture.